1. Field of the Invention
The present invention relates to an optical scanning apparatus and, more particularly, to an optical scanning apparatus suitable for an apparatus such as a laser beam, color laser beam, or multicolor laser beam printer having, e.g., an electrophotographic process for optically scanning the scanning surface of an image carrier such as a photosensitive or electrostatic recording body with a modulated laser beam from a laser source.
2. Related Background Art
In a conventional optical scanning apparatus such as a laser beam printer, the surface of the image carrier is optically scanned with a modulated laser beam to write image information, and the like.
FIG. 1 is a schematic view showing the main part of a conventional optical scanning apparatus.
Referring to FIG. 1, a beam emitted from a light source 101 such as a semiconductor laser is collimated by a collimator lens 102 to obtain a parallel beam. The parallel beam is focused through a stop 103 by a cylindrical lens 104 having a refracting power in only a sub-scanning direction and is linearly incident on a deflecting reflection surface 105a of an optical deflector 105 comprising a rotary polygon mirror or the like.
A beam reflected and deflected by the deflecting reflection mirror is guided to a scanning surface 107 through a scanning lens 106 to form a spot thereon. When the optical deflector 105 is rotated about a rotating shaft 105c by a driving means (not shown) such as a motor in a direction indicated by an arrow 105b, the scanning surface 107 is optically scanned in a direction indicated by an arrow 107a.
The stop 103 adjusts the spot shape of the beam on the scanning surface 107. The aperture of the stop 103 generally has a circular, elliptical, or rectangular shape.
In a conventional optical scanning apparatus in a laser beam printer or a laser beam copying machine, a semiconductor laser or an He--Ne laser is used as a light source. The optical amplitude intensity distribution of the laser beam emitted from such a laser source is a Gaussian distribution. In the conventional optical scanning apparatus, the laser beam having this Gaussian distribution is utilized to guide the beam on the image carrier surface through a collimator lens and a stop, thereby optically scanning this surface.
In the optical scanning apparatus shown in FIG. 1, when a stop having a circular or elliptical aperture is used, the optical amplitude distribution of a diffracted pattern of the beam passing through the stop becomes a Bessel distribution. The light intensity distribution of the beam upon optical scanning of the image carrier surface by the scanning lens becomes a conjugate product of the Bessel distribution by a Fourier transform operation. When the light intensity distribution of a beam incident on the stop is a Gaussian distribution, the light intensity distribution of the beam upon optical scanning of the image carrier becomes a Gaussian distribution.
Ideally when the light intensity distribution on the image carrier is a Bessel distribution, the beam spot diameter (i.e., a width at which the light intensity is decreased to 1/e.sup.2 with respect to a peak of 1) is minimized. That is, high-precision optical scanning can be performed.
In general, as part of the beam, however, is cut by the edge portion of the stop and a point at which the laser beam has a Gaussian light intensity distribution, the light intensity distribution of the beam on the image carrier surface becomes a state between the Bessel distribution and the Gaussian distribution.
When a stop having a rectangular aperture (including a square aperture) is used, diffracted light components are obtained in directions perpendicular to the sides of the stop, and a diffracted pattern becomes a sinc-functional distribution. In this case, the spot diameter becomes smaller than that of the Bessel distribution, but the light intensity of a side lobe increases.
The light intensity distribution of the beam becomes a multi-spot distribution or noise occurs in image formation by optical scanning.
When the stop having a rectangular aperture is used, the aperture of the collimator lens must match the diagonal length. For this reason, the aperture of the collimator lens tends to increase as compared with an arrangement using a stop having a circular or elliptical aperture.